1. Field of the Invention
The present invention relates to an alternator provided with a stator core composed of laminated steel plate having a base and a plurality of teeth projecting radially inwards from the base.
2. Description of the Related Art
FIG. 6 is a cross-section of a conventional automotive alternator, FIG. 7 is perspective view of the stator in FIG. 6, FIG. 8 is a perspective view of the stator core in FIG. 7.
This automotive alternator comprises: a case comprising an aluminum front bracket 1 and an aluminum rear bracket 2; a shaft 6 disposed in the case to one end of which a pulley 4 is secured; a Lundell-type rotor 7 secured to the shaft 6; fans 5 secured to both sides of the rotor 7; a stator 8 secured to the inner wall of the case; slip rings 9 secured to the other end of the shaft 6 for supplying electric current to the rotor 7; a pair of brushes 10 moving in contact with the slip ring 9; brush holders 11 accommodating the brushes 10; a rectifier 12 in electrical contact with the stator 8 for converting the alternating current generated in the stator 8 to a direct current; and a regulator 18 fitted over the brush holder 11 for adjusting the magnitude of the alternating current generated in the stator 8.
The rotor 7 comprises: a rotor coil 13 for generating a magnetic flux when an electric current is passed therethrough; and a pole core 14 disposed so as to cover the rotor coil 13, in which magnetic poles are produced by the magnetic flux generated by the rotor coil 13. The pole core 14 comprises a first pole core assembly 21 and a second pole core assembly 22 which are mutually interlocked. The first pole core assembly 21 and the second pole core assembly 22 are made of iron, and have claw-shaped magnetic poles 23, 24, respectively.
The stator 8 comprises: a stator core 15 through which a rotating magnetic field generated by the rotor 7 passes; and a stator coil 16 being a conductor wound around the stator core 15 through which a generated electric current flows.
The stator core 15 is produced by laminating a punched strip of steel plate having a plurality of teeth spaced equidistantly as the plate is curved into a spiral shape so that the ends of the teeth point towards the center. The stator core 15 comprises a base 30 and a plurality of teeth 31 projecting radially inwards from the base 30. Slots 32 are disposed between adjacent teeth 31 and a winding is wound into the slots 32.
In a vehicle alternator of the above construction, a current is supplied by a battery (not shown) through the brushes 10 and slip rings 9 to the rotor coil 13, whereby a magnetic flux is generated, giving rise to a magnetic field. At the same time, the pulley 4 is driven by the engine and the rotor 7 is rotated by the shaft 6, so that a rotating magnetic field is imparted to the stator core 15 and electromotive force is generated in the stator coil 16.
The magnetic flux A generated in the rotor coil 13, leaves the north-seeking (N) first pole core assembly 21 and enters the teeth 31 of the stator core 15 across an air gap between the rotor 7 and the stator 8. The magnetic flux A then passes through the base 30 and flows back across the air gap from adjacent teeth 31 to the south-seeking (S) second pole core assembly 22. The amount of magnetic flux, which determines the output of the alternator, depends on the magnetomotive force of the rotating magnetic field generated by the rotor 7 and the electromagnetic resistance of the electromagnetic circuit formed by the magnetic flux A. Consequently, if the magnetomotive force is the same, attaining a shape for the electromagnetic circuit with the smallest electromagnetic resistance possible becomes important.
In an automotive alternator of the above construction, since the magnetic flux A passing through the teeth 31 is diverted to the left and right at the roots of the teeth 31 and passes through the base 30, as shown in FIG. 9, the outer circumference B of the base 30 opposite the teeth 31 is not required for the electromagnetic circuit, and is even less desirable in order to lighten the stator core 15, and as a result, recesses 33 extending in the axial direction are formed in the outer circumference B. Moreover, the recesses 33 function as portions engaged by a jig when curving the strip of steel plate into a spiral.
In an automotive alternator of the above construction, the recesses 33 are formed in the stator core 15 in order to lighten the stator core 15 and to function as engaged portions for the jig, but a problem is that if the recesses exceed a predetermined size, electromagnetic resistance is increased by the constriction of the electromagnetic path reducing the amount of magnetic flux, thereby decreasing the output voltage of the alternator.
Furthermore, the stator core 15 is produced by curving and laminating a strip of steel plate into a spiral, but as shown in FIG. 10, the steel plate comprises a frame 40 and a plurality of tooth segments 41 extending perpendicularly from the frame 40. For that reason, when curving the steel plate, stress is concentrated at locations C joining the inside corner portions of the tooth segments 41 and the groove portions 42 formed on the outside of the frame 40, giving rise to large plastic deformations at the locations C, and so another problem is that the stator core 15 becomes a nonuniform polygon shape with sides formed by the base, leading to deterioration in the roundness of the inner circumference of the stator core 15, making the dimensions of the air gap between the rotor 7 and the stator core 15 nonuniform, whereby the magnetic flux density waveforms arising in the air gap are disrupted and power output decreases.